In various radio communication systems for WiMAX (Worldwide Interoperability for Microwave Access) (registered trademark), next generation PHS (Personal Handy-Phone System), LTE (Long Term Evolution) and the like, a communication technology in which both of a transmitting side and a receiving side include a plurality of antennas, that is, MIMO (Multiple Input Multiple Output) has been employed for improving throughput and frequency utilization efficiency.
Examples of MIMO to be employed in a downlink communication mode from a radio base station to a radio terminal include an STC (Space-Time Coding) base and an SM (Spatial Multiplex) base (for example, refer to Patent Literature 1 (Japanese Patent Laying-Open No. 2009-273186)).
According to the STC base, the radio base station places (i.e., codes) one signal stream on the basis of a certain rule with regard to time and space (antenna) and transmits the coded signal stream through a plurality of antennas. In WiMAX, this STC base downlink communication mode is called DL MIMO MATRIX-A.
On the other hand, according to the SM base, the radio base station multiplex-transmits a plurality of signal streams through a plurality of antennas at a single frequency. In WiMAX, this SM base downlink communication mode is called DL MIMO MATRIX-B.
Depending on conditions of a transmission path, the radio terminal is occasionally improved in throughput characteristic and area characteristic, further, frequency utilization efficiency by employing the DL-MIMO communication mode based on the space-time coding mode (DL MIMO MATRIX-A) or the radio terminal is occasionally improved in throughput characteristic and area characteristic, further, frequency utilization efficiency by employing the DL-MIMO communication mode based on the spatial multiplexing mode (DL MIMO MATRIX-B). Typically, the switchover from the space-time coding mode to the spatial multiplexing mode is performed in a case where the condition of the transmission path is favorable.